An essential element of modern mobile communications systems is the cellular telephone base station, also known as a “cell site.” The cell site includes one or more directional base station antennas aimed at a desired geographical area of coverage with coaxial cables connecting the antennas to base station radio equipment. The performance of a cell site is often limited by passive intermodulation (PIM) interference. PIM interference occurs when the high-power downlink signals transmitted by the base station antennas mix at passive, non-linear junctions in the RF path, creating new signals known as intermodulation products. When these intermodulation products fall in an operator's uplink band, they act as interference and reduce the SINR (signal to interference plus noise ratio). As the SINR is reduced, the geographic coverage and data capacity of the cell site is reduced.
It is well documented that loosely touching metal-to-metal surfaces can behave in a non-linear fashion and become sources of PIM interference when illuminated by high power RF (radio frequency) signals. Recently, it has been determined that loose metal-to-metal connections located behind base station antennas are also able to generate high levels of PIM interference. Even though this region is well outside the main beam of the antenna, enough RF energy is present in this region to excite non-linear objects and generate PIM interference. Based on field measurements it has been determined that loose metal-to-metal contacts located very close to base station antennas (within 1 wavelength of the carrier frequency) are more likely to generate high levels of PIM interference than loose metal-to-metal contacts located farther away (greater than 1 wavelength) from base station antennas.
A common source of loose metal-to-metal contact found in the region close to the base station antenna is metal brackets and associated hardware for supporting coaxial cables. Coaxial cables, typically ½-inch in diameter, are used to transfer RF signals between tower mounted radio equipment and the base station antenna. These cables need to be mechanically supported periodically along their length to prevent movement of the cable in the wind. The metal antenna mounting pipe close to the back of the base station antenna provides a convenient rigid surface to mechanically secure these coaxial cables. At operating frequencies at or below 2 GHz, this mounting pipe is typically located within 1 wavelength of the antenna within the zone of high PIM concern.
Two different methods for mechanically supporting coaxial cables are commonly found at cell sites. The first utilizes two plastic clamp blocks that fit around one or more coaxial cables. An example of this style cable support block is disclosed in Jobin et al., U.S. Pat. No. 5,794,897, which is incorporated by reference. A ⅜-inch or 10 mm diameter stainless steel threaded fastener is inserted into the support block pairs and stainless-steel hardware is installed to clamp the plastic block halves together on the threaded fastener. A steel interface bracket is often attached to one end of the threaded fastener using nuts and lock washers. The interface bracket is then secured to the antenna mounting pipe or other nearby metal members using a stainless-steel hose clamp. The hose clamp provides a convenient method for securing interface brackets to metal members since the hose clamp conforms easily to different shapes and is adjustable in length allowing it to fit around a wide variety of metal member sizes.
Another common system used for mechanically supporting coaxial cables uses metal “snap-in” style cable support hangers. The snap-in cable support hangers are made from thin “U” or “C” shaped stainless-steel members designed to wrap around individual coaxial cables. The hangers include locking features able to insert into round holes in supporting interface brackets. Once inserted, the locking features on the hanger expand outward to secure the cable to the interface bracket. A variation of this style cable hanger design includes a hole on one end of the hanger to accept an additional cable hanger. This enables multiple coaxial cables to be secured to a single interface bracket by stacking one hanger on top of another. Examples of this style cable hanger are disclosed in Paske, U.S. Pat. No. 6,354,543 and Feige, U.S. Pat. No. 8,439,316.
A problem with these conventional designs is that PIM can be generated at the metal-to-metal contacting surfaces between stacked metal snap-in style cable hangers and at the metal-to-metal contacting surface between the interface bracket and the snap-in hanger. Manufacturers such as Commscope have introduced plastic versions of their stackable snap-in style cable support hangers that eliminate the metal-to-metal contacting surfaces that generate PIM. These all-plastic snap-in cable hangers, however, introduce new problems. First, the all-plastic snap-in hangers are not as strong as the all-metal snap-in hangers. This limits the number of cables that can be reliably stacked on top of each other for a given support spacing. The all-plastic snap-in hangers are also prone to breaking, for example during installation when the plastic locking features are over-stressed due to misalignment.
A second problem with all-plastic snap-in hangers is that the plastic material used to produce these hangers is not able to bite into the cable jacket as effectively as all-metal snap-in hangers. This reduces the all-plastic hanger's ability to prevent longitudinal movement of the cable due to wind forces or due to gravity when the cable is oriented vertically.
A third problem with the existing all-plastic snap-in hangers is that they are not able to rotate freely at the hanger-to-hanger interface. Due to geometry constraints, the all-plastic hangers are only able to connect to one another in a fixed orientation. This requires all supported cables to be parallel to each other at the point of support. Cables that are not perfectly parallel stress the plastic supports, leading to breakage. Mechanical stress is also imposed on the RF cable, leading to cable deformation and reduced RF performance.
An improved low PIM snap-in style cable hanger is therefore needed to overcome the limitations of the existing alternatives.